This invention relates to aluminum alloy foils useful as cathode foils for electrolytic capacitors comprising an anode foil formed with a dielectric coating and a cathode foil having no dielectric coating and opposed to the anode foil with an electrolyte disposed therebetween.
Throughout the specification and the appended claims, the percentages are all by weight.
Various improvements have been made in anode foils to provide anode foils of greater capacitance and give an increased capacitance to electrolytic capacitors of the type mentioned above, while aluminum foils of low purity, namely about 99.3%, are usually used as cathode foils. However, the cathode foil, like the anode foil, must also have an increased capacitance for the fabrication of electrolytic capacitors with a higher capacitance. Aluminum foils having a purity of 99.5 to 99.8% and aluminum alloy foils prepared from aluminum and some other element, such as magnanese, useful as cathode foils have a greater capacitance than aluminum foils with a low purity of about 99.3%, but these foils still remain to be improved in capacitance.
Cathode foils of increased capacitance are prepared by etching the surface of a foil to form minute cavities therein uniformly with a high density and give an enlarged surface area to the foil. With aluminum foils of about 99.5 to 99.8% in purity, the foil must be etched to excess in order to obtain the desired capacitance. This results in an excessive corrosion weight loss, locally creating pits in the foil, and eventually gives a reduced capacitance and greatly impaired mechanical strength to the foil. Difficulties are also encountered with foils of aluminum-manganese alloy in forming minute cavities uniformly with a high density by etching its surface. When etched excessively, the foil is subject to local pitting, consequently failing to have the desired capacitance.